1. Field of the Invention
The present invention relates to an image forming apparatus and a liquid control method, and more particularly, to a liquid droplet control technology for controlling the amount of dissolved gas in liquid droplets used to form an image on a recording medium, and maintaining the liquid droplets in a desirable state.
2. Description of the Related Art
In recent years, inkjet printers have come to be used widely as data output apparatuses for outputting images, documents, or the like. An inkjet printer forms data on recording paper by driving recording elements (nozzles) of a recording head in accordance with data, thereby causing ink to be discharged from the nozzles. Discharge devices for causing discharge of the ink include devices using PZT actuators, or the like, which apply a pressure wave to a pressure chamber connected to a nozzle, and devices using a heat source which heats ink contained in an ink chamber (pressure chamber) and thus generates bubbles in the ink. Ink pressurized by operating a discharge device of this kind is discharged from the nozzles and data, such as an image, is formed on a recording medium.
In an inkjet printer, if air bubbles become mixed into the ink inside the recording head, then there is significant loss of the pressure applied to the ink by the actuator, and this can lead to discharge abnormalities, such as abnormalities in the amount of ink discharged or the direction of discharge, discharge failures, and the like. Discharge abnormalities of this kind affect image recording and consequently cause a marked decline in the resulting image quality. Therefore, it is possible to maintain the quality of the recorded image by detecting discharge abnormalities swiftly and eliminating the causes of the discharge abnormalities.
A known method for preventing the occurrence of air bubbles inside a print head (pressure chamber), which are a cause of discharge abnormalities as described above, uses a so-called deaerated ink which has a reduced amount of gas dissolved in the ink.
Japanese Patent Application Publication No. 2000-190529 discloses a method for controlling the amount of dissolved gas in a liquid in an inkjet apparatus, an inkjet recording apparatus and a color filter manufacturing apparatus, according to which the amount of dissolved gas in unused ink that has been circulated within an inkjet head is measured, and the ink is circulated and dissolved gas is removed from the ink if the measurement value exceeds a prescribed value, in such a manner that the amount of dissolved gas in the ink is equal to or less than the prescribed value.
Furthermore, in the inkjet printer and the deaeration method for an inkjet printer described in Japanese Patent Application Publication No. 11-20194, a tube provided in the flow path of the ink is reduced in pressure, thereby removing air contained in the ink inside the tube.
In the inkjet recording apparatus described in Japanese Patent Application Publication No. 11-48491, a deaerating device comprising a hollow fiber filter is provided between an ink container and a recording head, in such a manner that ink passes through the deaerating device when supplied to the recording head.
However, if a deaerator and a dissolved oxygen meter for measuring the amount of dissolved gas in the ink are disposed in this sequence before a print head, from the upstream side of the ink flow path, then since there is no device for measuring the dissolved gas downstream of the dissolved oxygen meter, a problem arises in that the amount of dissolved gas in the ink inside the print head will remain as it is. The same also applies to systems which do not use a device, such as a dissolved oxygen meter, for measuring the dissolved gas in the ink.
In the method for controlling the amount of dissolved gas in a liquid inside an inkjet apparatus, the inkjet recording apparatus, and the color filter manufacturing apparatus according to Japanese Patent Application Publication No. 2000-190529, since there is a long distance between the dissolved oxygen meter that measures the amount of dissolved gas in the ink and the deaerator, then in cases where the apparatus is printing at low duty which does not consume ink, or where ink is held for a long time inside the inkjet head, or the like, it may occur that the amount of dissolved gas in the ink exceeds a specified value when it reaches the dissolved oxygen meter, even though the amount of dissolved gas does not exceed the specified value in the inkjet head section, and hence the apparatus halts printing and enters deaerating mode, thus causing time loss.
Furthermore, it is supposed that the deaerator used here has sufficiently high capacity with respect to the flow rate of the ink, but if the ink remains stationary for a long period of time, then the deaeration rate will exceed the specified value. Moreover, this patent publication does not provide any description relating to a multiple head in which a plurality of print heads are disposed. For example, if a deaerator is provided at one point of an ink supply path (in other words, before branching of the path), then it may not be possible to judge conditions accurately, due to differences in the use duty of the respective print heads. On the other hand, if deaerators are positioned after branching in a multiple head, and a dissolved oxygen meter is positioned downstream of the print heads on the circulation side, then the number of deaerators will increase and costs will rise.
Furthermore, in the inkjet printer and deaeration method for an inkjet printer according to Japanese Patent Application Publication No. 11-20194, and the inkjet recording apparatus according to Japanese Patent Application Publication No. 11-48491, a measuring device, such as a dissolved oxygen meter, for measuring the amount of dissolved gas in the ink is not provided, and therefore it is not possible to ascertain the amount of dissolved gas in the ink inside the apparatus.